Carbamates with —NHC(O)O— structural unit are important raw materials for a variety of polymers (e.g., polyurethanes) used in foams, coatings, adhesives, plastics and fibers. They find application as herbicides, fungicides and pesticides in agrochemical industry (e.g., CARBARYL, CARBOFURAN, PROPOXUR, DIOXACARB, AMINOCARB etc.) and drug intermediates in pharmaceutical industry (e.g., secondary amyl carbamate, trichloroethyl carbamate, physostigmine, carbachol etc.).
Carbamates are conventionally produced by phosgene/isocyanate technology wherein aromatic or aliphatic amine is reacted with phosgene, which is then treated with an alcohol to obtain the corresponding carbamates. This process using phosgene and isocyanate is highly toxic and hence, unsafe. Another incentive to eliminate phosgene is the economic penalty incurred because the chlorine content of phosgene is wasted and converted into NaCl. Caustic soda is consumed in the conversion and the disposal of waste salt solutions presents ecological problems in itself.
Production of carbamates by reductive carbonylation route using noble metal catalysts is another alternative but it is economically not viable; only one-third of CO could be utilized effectively and the separation of CO and CO2 increases the operation cost.
U.S. Pat. No. 5,502,241 which deals with the preparation of alkyl carbamates in particular methyl methyl carbamates by reacting methyl amine or N,N′ dimethyl urea with CO, an oxidizing agent and a mono alcohol in the presence of a platinum-based catalyst and a quaternary ammonium halide as a promoter. U.S. Pat. Nos. 4,304,922; 4,297,560; 5,194,660 and 5,502,241 also describe the above said oxidative carbonylation route. High yields of carbamates are achieved with this route. But the method of preparation is hazardous as it involves handling of CO+O2/air mixtures at harsh conditions (50-400 bar; 443 K). Eco-friendly routes for the preparation of carbamates are, therefore, highly desirable.
Methoxycarbonylation of amines using dimethyl carbonate (DMC) as methoxylating agent was proposed as a phosgene-free route (Tetrahedron Letters Year 1986, Vol. 27 page 5521). However, separation of methanol-DMC azeotrope is an expensive operation in this process.
Carbamates can also be synthesized by the Hoffmann rearrangement of amides, reaction of chloroformates and amines etc (Tetrahedron Letters Year 1997, Vol. 38, 8878; Year 1998, Vol. 39, 3259).
Among the several phosgene-/isocyanate-free alternative routes, reaction of primary amines with CO2 and organic halide is the most promising high yielding route (J. Chem. Soc. Chem. Commun. Year 1994, page. 699; Tetrahedron Year 1992, vol. 48, page 1515; U.S. Pat. Nos. 6,528,678; 6,399,808). In addition to the advantageous feature of not being hazardous, the synthetic route contributes to the issue of utilization of “greenhouse effect gas” CO2 and environmental-clean-up. Generally, strong organic bases, crown ethers and onium salts in homogeneous phase stabilize the carbamate anion and catalyze the synthesis of carbamates (Chem. Rev. 2004; J. Org. Chem. Year 1995, vol. 60, 2820). There have been reports on the use of ionic liquids and solids like CsCO3 and K2CO3 for this reaction as catalysts (Tetrahedron Year 2002, Vol. 58, page 3329; J. Org. Chem. Year 2001; Vol. 66, page. 1035; Organic Letters Year 2000, Vol. 2, page 2797). However, due to their low activity very large amounts of such catalysts (almost equal to the quantity of the substrate) had to be used at long reaction times. Moreover these catalysts require large amounts of quaternary ammonium salt promoters to enhance carbamation while suppressing N-alkylation (U.S. Pat. No. 6,399,808). Other reports that deal with the synthesis of organic carbamates include U.S. Pat. No. 6,566,533 (describing the production of heterocyclic carbamates from aza-heterocycle compound from carbon dioxide); U.S. Pat. No. 5,688,988 (dealing with a process for the production of aromatic urethane by reacting aromatic amines and organic carbonates in presence of Zn and/or copper carbonates/hydroxide); U.S. Pat. No. 4,156,784 (describing carbamates manufacturing by reaction of alcohols with urea in prescence of ion exchangers containing Ni) and U.S. Pat. No. 4,415,745 (describing a process for preparation of aromatic carbamates from isocyanates).
Recently, Srivastava et al., reported the use of titanosilicate molecular sieves and zeolite-encapsulated metal phthalocyanine complexes for this reaction (Srivastava et al. Catal. Lett. Vol. 97, Year 2004, pp. 41-47). Although these catalysts could be reused in recycling experiments high yields of carbamates could be obtained only when solvents like N,N-dimethyl formamide are used in the reaction.
The present invention is a “green” process carried out in the presence of a solid catalyst at moderate temperatures and CO2 pressures. The catalyst could be separated easily by simple filtration and reused. Most importantly, the catalyst is highly efficient and only a small amount of it is needed unlike the prior art solid catalysts. The present invention utilizes an ordered, mesoporous, modified-silica-based bifunctional catalyst;
Mesoporous silica surface is modified with a Lewis acid metal ion preferably tetrahedral Ti4+ ions (by grafting) as well as with organic base preferably adenine (by anchoring). Site isolation and synergism between the Lewis acid metal ion and anchored organic base is the major cause for the superior activity of the catalyst of present invention. It has been observed that the solid catalyst of the present invention exhibits good activity with high carbamate selectivity even in the absence of a solvent.